Semiconductor nanocrystallites (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller.
Bawendi and co-workers have described a method of preparing monodisperse semiconductor nanocrystallites by pyrolysis of organometallic reagents injected into a hot coordinating solvent (J. Am. Chem. Soc., 115:8706 (1993)). This permits temporally discrete nucleation and results in the controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of the crystallites from the growth solution provides crystallites with narrow size distributions. The narrow size distribution of the quantum dots allows the possibility of light emission in very narrow spectral widths.
Although semiconductor nanocrystallites prepared as described by Bawendi and co-workers exhibit near monodispersity, and hence, high color selectivity, the luminescence properties of the crystallites are poor. Such crystallites exhibit low photoluminescent yield, that is, the light emitted upon irradiation is of low intensity. This is due to energy levels at the surface of the crystallite which lie within the energetically forbidden gap of the bulk interior. These surface energy states act as traps for electrons and holes which degrade the luminescence properties of the material.
In an effort to improve photoluminescent yield of the quantum dots, the nanocrystallite surface has been passivated by reaction of the surface atoms of the quantum dots with organic passivating ligands, so as to eliminate forbidden energy levels. Such passivation produces an atomically abrupt increase in the chemical potential at the interface of the semiconductor and passivating layer (See, A. P. Alivisatos, J. Phys. Chem. 100:13226 (1996)). Bawendi et al. (J. Am. Chem. Soc., 115:8706 (1993)) describe CdSe nanocrystallites capped with organic moieties such as tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO) with quantum yields of around 5–10%.
Passivation of quantum dots using inorganic materials also has been reported. Particles passivated with an inorganic coating are more robust than organically passivated dots and have greater tolerance to processing conditions necessary for their incorporation into devices. Previously reported inorganically passivated quantum dot structures include CdS-capped CdSe and CdSe-capped CdS (Tian et al., J. Phys. Chem. 100:8927 (1996)); ZnS grown on CdS (Youn et al., J. Phys. Chem. 92:6320 (1988)); ZnS on CdSe and the inverse structure (Kortan et al., J. Am. Chem. Soc. 112:1327 (1990)); and SiO2 on Si (Wilson et al., Science 262:1242 (1993)). These reported quantum dots exhibit very low quantum efficiency and hence are not commercially useful in light emitting applications.
M. A. Hines and P. Guyot-Sionnest report the preparation of ZnS-capped CdSe nanocrystallites which exhibited a significant improvement in luminescence yields of up to 50% quantum yield at room temperature (J. Phys. Chem. 100:468 (1996)). However, the quality of the emitted light remained unacceptable because of the large size distribution (12–15% rms) of the core of the resulting capped nanocrystallites. The large size distribution resulted in light emission over a wide spectral range. In addition, the reported preparation method does not allow control of the particle size obtained from the process and hence does not allow control of color.
Danek et al. report the electronic and chemical passivation of CdSe nanocrystals with a ZnSe overlayer (Chem. Materials 8:173 (1996)). Although it might be expected that such ZnSe-capped CdSe nanocrystallites would exhibit as good as or better quantum yield than the ZnS analogue due to the better unit cell matching of ZnSe, in fact, the resulting material showed only disappointing improvements in quantum efficiency (≦0.4% quantum yield).
Thus there remains a need for semiconductor nanocrystallites capable of light emission with high quantum efficiencies throughout the visible spectrum, which possess a narrow particle size (and hence with narrow photoluminescence spectral range).
It is the object of the invention to provide semiconductor nanocrystallites which overcome the limitations of the prior art and which exhibit high quantum yields with photoluminescence emissions of high spectral purity.